1. Field of the Invention
The present invention relates to a cordierite ceramic filter to be used at high temperatures, and a method of producing the same.
2. Related Art Statement
Recently, as for porous ceramic filters, various types of porous honeycomb filters have been known, wherein filtration is performed by passing a fluid such as gas through partition walls of a honeycomb structural body made of cordierite. Since cordierite has an excellent heat resistance as compared with other materials, the cordierite honeycomb is used as a catalyst carrier in automobiles and so on, which needs an excellent thermal shock resistance.
In the known cordierite ceramic filter especially used for collecting fine particles in an exhaust gas supplied from a diesel engine, since a filter regeneration is performed by firing the collected fine particles in the ceramic filter, the cordierite ceramic filter is not used stably for long time period due to a deterioration or melt down thereof. In order to eliminate the drawbacks mentioned above, it is disclosed that a filter regeneration is performed by using a reverse pulse air. However, the filter regeneration method mentioned above has various drawbacks such that filter regeneration efficiency is not sufficient, an apparatus for performing the filter regeneration method has insufficient mechanical strength for being assembled in automobiles, and thus a total system for performing the filter regeneration method must be large to improve the insufficient efficiency and mechanical strength mentioned above.
The known cordierite ceramic filter is produced by mixing raw materials such as talc, kaolin, alumina and so on with forming agents to obtain a ceramic batch, extruding the ceramic batch to obtain a honeycomb structural green body, firing the honeycomb structural green body to obtain a honeycomb structural body, and sealing both ends of the honeycomb structural body in a checker pattern, wherein, if one end of a flow passage is closed, the other end of the flow passage is open. Japanese Patent Laid-open Publication No. 61-129015 (JP-A-61-129015) discloses a method for improving the filter efficiency such that pores existent in a surface of a filter partition wall are controlled to satisfy a ratio of (the number of small pores having a diameter of 5-40 .mu.m):(the number of large pores having a diameter of 40-1000 .mu.m) in a range of 1:51:40. However, this method is developed for filter regeneration by firing but not for filter regeneration by reverse pulse air. Therefore, even if filter regeneration by using reverse pulse air is applied to the filter disclosed in JP-A-61-129015, sufficient filter regeneration cannot be performed.
As for a filter to which filter regeneration by using reverse pulse air is applied, Japanese Patent Laid-open Publication No. 63-31517 (JP-A-63-31517) discloses a method of producing a ceramic filter by mixing one or more than two kinds of ceramic fine powders selected from the group of cordierite, .beta.-spodumene, aluminum titanate, mullite, zirconia, silicon carbide and silicon nitride with clay or frit as a binder to obtain ceramic batches, and firing the ceramic batches. However, the ceramic filter formed according to the method mentioned above cannot obtain sufficient mechanical strength and cannot perform filter regeneration by using reverse pulse air. Therefore, the ceramic filter mentioned above cannot be preferably used in automobiles which need a long particle operating time.
In order to obtain a ceramic body having an isotropic thermal expansion, Japanese Patent Publication No. 60-2272 (JP-B-60-2272) discloses a method of producing a cordierite body by mixing predetermined synthesized cordierite powders having an undetermined shape and raw materials for cordierite generation to obtain mixtures having a theoretical cordierite composition, forming the mixtures to obtain a formed body, and sintering the formed body. This method is for a catalyst carrier, and thus it is effective in providing the ceramic body with excellent heat resistivity. However, it is not sufficient for the ceramic filter which needs an excellent trapping efficiency, since it does not disclose a porosity and a particle size of cordierite powders.
Japanese Patent Laid-open Publication No. 3-68411 (JP-A-3-68411) discloses a method of producing the ceramic filter showing a small pressure loss and a sufficient mechanical strength for a real use by using densified cordierite having a suitable particle size as aggregates. However, according to the method mentioned above, in order to densify the aggregates, it is necessary to fire cordierite powders having high porosity and to add LAS raw materials. Therefore, producing steps increase extraordinarily.